1. Field of the Invention
The present invention generally relates to electronic systems, and more specifically to systems using electromagnetic transponders, that is, transceivers capable of being interrogated in a contactless and wireless manner by a read and/or write terminal.
2. Discussion of the Related Art
Many communication systems are based on a modulation of an electromagnetic field generated by a terminal. They range from the simplest electronic tag used as a theft-prevention device to more complex systems where a transponder intended to communicate with the terminal having it in its field, is equipped with calculation functions (electronic purse, for example) or data processing functions.
Electromagnetic transponder systems are based on the use of oscillating circuits comprising a winding forming an antenna, on the transponder side and on the terminal side. Such circuits are intended to be coupled by a near magnetic field when the transponder enters the field of the terminal. The oscillating circuits of the terminal and of the transponder are generally tuned to the same frequency corresponding to the excitation frequency of the oscillating circuit of the terminal.
In most cases, transponders have no autonomous power supply and extract the power supply necessary to their circuits from the high-frequency field radiated by the antenna of the terminal.
The present invention more specifically applies to transponders equipped with a processing unit capable of performing calculations, for example, on request of a terminal having the transponder in its field. This type of transponder is especially involved as soon as cryptographic calculations need to be implemented, for example, to authenticate or cipher a transmission. Now, in a given position with respect to a terminal, a transponder cannot currently evaluate the maximum current that its circuits can consume.
When a transponder has to perform calculations, it would be desirable to know in advance whether the voltage that it recovers from the field radiated by the terminal enables it to perform these calculations. It would in particular be desirable for a transponder not to start a cryptographic calculation that it cannot complete. This may also be used to manage priorities between processings according to the available power.
The power transfer between the terminal and the transponder depends on the coupling between the terminal and the transponder. This coupling, which is inversely proportional (non-linear) to the distance between the terminal and the transponder, conditions the amplitude of the voltage recovered by the transponder.
When a transponder is dedicated to a type of terminals, it can be envisaged to size the circuits to optimize the coupling. It can further be envisaged to determine, for a given calculation, the amount of power needed by the transponder, and thus the voltage that it needs to recover to obtain this power. However, transponder systems generally provide for a given transponder to be usable in cooperation with a large number of terminals which have different features.